Atomic-Scale Mechanisms of Superionic Conduction and Interfacial Phenomena in Sulfide Electrolytes for Lithium Batteries

Rechargeable solid-state lithium metal batteries (SSLMBs) offer tremendous promise as energy-dense and safe energy storage solution for electric vehicles, mobile computing, and portable electronics. Particularly, SSLMBs utilizing sulfide electrolytes (e.g., prototypical Li₇PS₆argyrodites) have garnered a lot of attention owing to their remarkable Li-ion conductivity (~10^-3 S/cm), elastic stiffness (~30 GPa), and low flammability. Nevertheless, sulfide electrolytes remain far from commercial SSLMBs due to longstanding issues with slow Li-ion conduction under ambient conditions, and parasitic side reactions at the Li-anode. These problems stem from a lack of fundamental understanding of the atomic-scale processes underlying ion conduction, charge transport, structural evolution, and interfacial reactions (e.g., dendrite growth, electrolyte decomposition, etc.) in SSLMBs. Using halogen-doped Li-argyrodite as a representative system, we address this crucial knowledge gap by integrating density functional theory (DFT) calculations, machine learning (ML), and <i>ab initio</i>/classical molecular dynamics (AIMD/ CMD) simulations. Specifically, using AIMD simulations, we find that fluorine-containing argyrodite electrolytes that simultaneously offer enhanced (a) Li-ion conduction facilitated by unique Li-disorder induced by fluorine and other halogen co-dopants, and (b) stability against the Li-anode owing to the formation of a stable solid-electrolyte interface containing conductive species (Li₃P), alongside LiCl and LiF. Furthermore, we employed a large database of energies, atomic forces, charges, elastic/thermal properties, energetics/pathways for key reactions, obtained using DFT calculations to train an accurate reactive force field (ReaxFF) for Li/P/S systems. The newly developed ReaxFF can accurately capture (a) structure, energetics, and dynamics across multiple length scales, (b) thermal properties, (c) interfacial reactions, and synthesis pathways, (d) mechanical properties, (e) ion transport, (f) and atomistic response to environment (temperature, pressure, etc.) Importantly, molecular dynamics simulations based on the newly developed model reveal interfacial reactions at electrified interfaces. We will discuss these results in the context of accelerating the design of novel solid-state electrolytes for long-lived, stable, and high-energy-density lithium batteries.